Wide band transmitting and receiving system



Nov. 24, 1942.-A

DE WITT R. GODDARD WIDE BAND TRANSMITTING AND RECEIVING SYSTEM I Filed March v, 1941 Patented Nov. 24, 1942 ING SYSTEM De Witt Rugg Goddard, Riverhead, N. Y., assignor to Radio Corporation of America., a corporation of Delaware Application Maren 7, 1941, serial Ne. 382,175

' (oigan- 6) Claims.

In certain systems in use for transmitting and receiving wide band` widths such as are required for television the tuned circuits comprisinglthe tank circuits of the transmitter are given a very low Q in order not to discriminate too greatly against the higher modulation frequencies. This does not permitan impedance of over a few hundred ohms to be built up in the. tank circuits, with the result that little power can be realized with` the presentL transmitting tubes available. The use of the low Q circuits necessary to accommodate the wide band of television signals as a consequence results in rather poor power eiciencies.

Certain wide band width receivers such as are used for television have asimilar trouble. The intermediate frequency coupling transformers in order to have a reasonably flat characteristic kover the required band width of several megacycles must be loaded to such an extent that with the receiving tubes at present available, a low gain per stage is all that can berealized.

, My system provides a method of radiating almost any reasonable power at several times the band Widths already in use in television systems. Also in my system the diiculty of emitting large amounts of power at frequencies of several hundred megacycles is materially reduced.

In describing my invention Areference Will be made to the attached drawing wherein:

Figure 1 illustrates schematically a wide band transmitter such as, for example, a television transmitter arranged in accordance with. my invention; f

Figure 2 illustrates schematically a wide band receiver such as, for example, atelevision signal receiver arranged in accordance with my invention; while Figure 3 illustrates admodication of the arrangement of Figure 2.

My invention comprises a transmitting and receiving systemin which tubes at present available may be used with greater efliciency and almost any band widthmay be handled.

A transmitter arranged in accordance with my invention consists primarily of a system in which `carrier at 60 megacycles with up to ,3 megacycles of'modulation, that is, modulated energy covering about Gfmegacycles.- To simplify the description let us further assume that but one set of side bands is to be radiated, let us say on the high frequency side of the carrier. Now, referring to' Figure La high:l frequency oscillator l of low power is modulated by, say television signals by way of the modulator l2 in the customary fashion. The output of this oscillator which may be of low power is fed to the nonlnductive resistor R. This canall be accomplished without difliculty, and in doing so high gain, low capacity receiving tubes may be used. Across R. appears the 60 megacycle carrier together with a complete' set vof upper and lower side bands. Across R isV connected radio frequency band-pass filter #I passing frequencies from 60.0 megacycles to 61.0 megacycles. 'Ihis filter preferably passes the" 60.0 megacycle carrier and all the upper side bands between 60.0 and 61.0 megacycles. Also across R is connected filters #2 and #3" passing frequencies between 61.0 and 62.0 megacycles and 62.0 and 63.0 megacycles respectively. The outputs of these filters pass to power amplifiersv Nos. P. A. I', y2, and 3 as shown, These power yamplifiers radiate their power through` antennas l, 2 and 3,Y respectively.

The signal that is' passed into the ether from the above arrangement is no different from that of a single transmitter radiating 3 megacycles of modulation on a 60 megacycle carrier with the vlower side band suppressed. However, the above system would radiate three times the power and each power amplifier isV required to handle but 1 megacycle of modulation. In practice greater efciency than heretofore obtainable is realized because, due to' the narrower bandjof frequencies passed by each channel, higher impedances may be built up in the transmitter filter and amplifier tank circuits. It is not necessary to make the i'llters divide the modulation band equally. As most of the power is concentrated the carrier and the side bands close to it. it is in some cases pref,- erable to have lter #l pass, say, 60 to 60.5 megacyc1es,.lter #2- pass 60.5 'to 61.5 megacycles and filter #3 pass 61.5 to 63 megacycles. Aniarrangementor this nature provides a maximumy of efiiciency.

It will be, understood that in selecting a modulation frequency bandy of three megacycles applicant doesl not thereby limit his arrangement to theeuse of a band of modulation of this width.

f My invention is applicable toa narrower modulation band and is particularly applicable to a Wider modulation band such as, for example, a 9 megacycle kband of videov signals used in television. Theneed of my arrangement grows with the mo-dulation bandwidth.

Where a 9 megacycle modulation band is used the filters it I, #2 and #3 may each pass one-third of the band covered by the carrier and one of its side bands, or, as in the example given above, the iiltersv may vhave widthsl which increase as the frequencies passed thereby depart from the carrier frequency.

In a modification unit Ifmay include, in addition to the modulator, a selective system for selecting out the carrier and one side band and sup- Preferably in these filters I use` Y,

vto its first detector (Det. #L

Another arrangement is shown in Figure 3. Here three` antennas 40, 42 and 44 are connected vto three radio frequency amplifiers RF #L RF #2 and RF #3, each of which connects #2 and #3) which is also fed from a common oscillator 5D.

' The output of each of these detectors is fed to ranged with such reactances onultra -high fre-i quencies. Preferably I use a'filter that does not cause serious phase distortion, and of course, the filters have to overlap exactly the correct amount. The filters may be in several sections with opposed phase distortion characteristics which 'com'- pensate.

Another item of importance is the location of the antennas. These should have exactly the same geographical location in order to present a uniform field without phase discrimination between the three component parts. This effect can be accomplished in'two ways, either a single antenna may be substituted for the three shown in Figure l and each of the three power amplifiers P. A. #L #2 and #3 coupled to it or three antennas, #L #2 and #3, eachconsisting of a plurality of radiating members, all three being arranged about a single vertical axis, may be used.

It should be mentioned that the original sigvnal need not necessarily be split up into three components. It may be split into two or four components, or as many as economy and power requirements dictate.

The receiving system to be outlined is in theory similar to the transmitter in so far as it splits the signal to be received into two or more parts, deals with each part independently and later combines them to produce the desired result. This has several advantages in receiver design. In the first place the problem of` producing an intermediate frequency amplifier having high gain and a substantially fiat band-pass of 3 megacycles is diicult with the tubes now available. An intermediate frequency amplifier to pass 4 or 5 megacycles would present a still greater problem as the gain per stage would be low due to the low plate irnpedances that can be built up.'

Figure 2 shows lan arrangement'in which the antenna and oscillator 26`feed the first detector to produce the intermediate frequency output. This output is passed to the input of three'intermediate frequency amplifiers, IF #L #2 and #3. Each intermediate frequency amplier amplifies a part of the total received signal and the output of the three amplifiers passes to', a single second detector or demodulator 36. Here as in the transmitters the several amplifier circuits each are subjected to a narrower band of frequencies and as a consequence the circuits may be of higher Q and the amplifiers operate'more eiciently and at higher frequencies.

If we assume the signal already described is to be received, then .intermediate frequency amplifier #l maybe adjusted to amplify the band an intermediate frequency amplier, IF #L #2 and #3.y k'Ihe output of these three intermediate frequency amplifiers feeds a common demodulator 56 from which is delivered the required modulation. The arrangement of filter band pass frequencies may be the same is in the receiver of Figure 2. 'Ihis arrangement, however, provides for the utilization of three antennas, each of vwhich operates atimproved efficiency due to the narrower band of response required of it. This isparticularly true where each antenna is a directive antenna. The antenna locations are of phase limitations as the antennas are phased up for any specific signal by adjusting the length of the transmission lines between the antennas and the radio frequency units.

It might be added that of course ordinary receivers may be fused with the special transmitter outlined or an ordinary transmitter used with the above-described receiver. Also, in the case of transmission to a single receiving station transmitting antennas may be spaced any desired distance and their respective phases adjusted by varying the length of the transmission line feeding the antennas.

I claim: f

l. In a signalling system, a source of wave energy,v a source of signalling potentials covering a wide range of frequencies, low power modulating means for modulating the wave energy/in accordance with said potentials, an impedance coupled to said last-named means, a plurality of rfilters having overlapping band pass characteristics and having inputs and outputs, means coupling the inputs of said filters in shunt to said impedance, a power amplifier coupled'to the output of each filter and utilizing means coupled to the power amplifiers.

2. In a system for demodulating a wind band of high frequency wave energy comprising a carrier and side band frequencies, a plurality of filters each passing a different band of said high frequency wave energy, said filters together-passing the entire band of frequencies, a'source of oscillations, a frequency converter coupled to each filter, a coupling between each converter and said source of oscillations forheterodyning the bands of wave energy passed by the-respective filters to corresponding bands of wave energy of lower frequency, la filter for'passing a-different band of frequencies coupledjto each of said frequency converters and signal demodulating means coupled to all of said last named filters.

3. In a signallingv system, a source of wave energy of carrier wave frequency, a source of signals covering a wide range of frequencies, a low power modulator for modulating the wave energy in accordance with said signals, said modulator having an output, a resistance connected to said output, `a plurality of wave filters having overeachiilter.

lapping band-pass characteristics and having inputs and outputs, connections coupling the inputs of said filters in shunt to' said resistance, and wave amplifying means coupled to the outputy of VDE wrr'r VRuder GODDARD. 1 

